Hardware that needs maintenance can cause a door to sag. Depending on how frequently your front or back door gets used your door hardware gets open, shut, and slammed. Inspect your exterior door hinges to make sure they operate properly. Quite frequently people find that the door hardware is loose, a screw is missing or the hinge is not properly aligned. If you have a squeaky door your door may need lubrication.
Get a screw driver and tighten any loose bolts and screws and lubricate hinges with oil. Make sure to wipe the excess oil up quickly so that it won’t drip and make a mess that can be tracked throughout your house.